Diet Demystified: Why We Overeat

As Americans begin the process of breaking their New Year's resolutions — sure, one king-sized Kit Kat won’t hurt anyone — they can forgive themselves with a consolation: Hormones may be to blame.

In a new study, which was published online Dec. 24 in the journal Biological Psychiatry, researchers have found that the hormone ghrelin causes mice to search out food — even when they weren't hungry.

"We've shown that ghrelin affects behaviors related to eating and that it may consequently lead to overeating," said study researcher Dr. Jeffrey Zigman, assistant professor of internal medicine and psychiatry at University of Texas Southwestern Medical Center.

The results could apply to us, as we also have this hormone, which has been shown in the past to activate the same brain regions excited by cocaine in humans.

In the experiments, Zigman and colleagues trained mice to remember that a room with striped walls held a high-calorie treat, while a chamber with gray walls contained a low-calorie snack. After the training, satiated mice were given the opportunity to wander to whatever room they wanted. Those mice that were injected with a dose of ghrelin preferred the room known for calorie-infused goodies, even if the room was empty of food. Mice that were ghrelin-free had no preference.

"The mice's behavior had nothing to do with eating," Zigman said. "Their behavior was linked to obtaining the more pleasurable thing."

Ghrelin is not the only factor involved with overeating, however. Here are other reasons some folks just can't stop chowing down:

Genetics: A 2003 study from Imperial College London revealed that people who carried the gene GAD2 were more likely to be obese. GAD2 speeds up the production of a neurotransmitter in the brain, which in turn stimulates us to eat.

Dopamine: In a study published in 2007 in the journal Behavioral Neuroscience, participants with low levels of the neurotransmitter dopamine were more likely to overeat in an attempt to stimulate pleasurable feelings.

Larger portions: Genetics aside, all people are induced to eat more if they are given larger portions. A 2005 Cornell University study found when moviegoers were served stale popcorn in big buckets they ate 34 percent more than those given the same stale popcorn in medium-sized containers.

Creatures of habit: If you always eat a box of Milk Duds at the movie theatre, there’s a good chance when you go to see "Avatar" you'll order some, even if you aren’t hungry.

Finally, there's the problem of limited willpower. If we ask the brain to do too many things at once, we simply won’t have the energy left to either stop ourselves from overeating (ghrelin or no ghrelin) or from imbibing in other activities we find rewarding, according to a 1999 study by Stanford University's Baba Shiv.